WindowEventHandlers.onbeforeunload

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句法
范例
注意事项
规范
浏览器兼容性
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onbeforeunload 特性为 WindowEventHandlers 混合 EventHandler 为处理 beforeunload events. These events fire when a window is about to unload its resources. At this point, the document is still visible and the event is still cancelable.

注意： To combat unwanted pop-ups, some browsers don't display prompts created in beforeunload event handlers unless the page has been interacted with. Moreover, some don't display them at all.

句法

window.addEventListener("beforeunload", function(event) { ... });
window.onbeforeunload = function(event) { ... };

Typically, it is better to use window.addEventListener() 和 beforeunload event, instead of onbeforeunload .

范例

This example prompts the user before unloading.

The HTML specification states that authors should use the Event.preventDefault() method instead of using Event.returnValue to prompt the user.

window.addEventListener('beforeunload', function (e) {
  // Cancel the event
  e.preventDefault(); // If you prevent default behavior in Mozilla Firefox prompt will always be shown
  // Chrome requires returnValue to be set
  e.returnValue = '';
});

Guarantee the browser unload by removing the returnValue property of the event

window.addEventListener('beforeunload', function (e) {
  // the absence of a returnValue property on the event will guarantee the browser unload happens
  delete e['returnValue'];
});

注意事项

When your page uses JavaScript to render content, the JavaScript may stop when leaving and then navigating back to the page. You can bind to window.onbeforeunload to prevent the browser from fully caching the page. If you do so, JavaScript in the page will be triggered on the subsequent return visit and update the content as desired.

规范

The event was originally introduced by Microsoft in Internet Explorer 4 and standardized in the HTML5 specification.

规范	状态	注释
HTML 实时标准 The definition of 'onbeforeunload' in that specification.	实时标准
HTML 5.1 在该规范中的 GlobalEventHandlers 定义。	推荐
HTML5 在该规范中的 GlobalEventHandlers 定义。	推荐

浏览器兼容性

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. 更新 GitHub 上的兼容性数据

	桌面						移动
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`onbeforeunload`	Chrome 1	Edge 12	Firefox 1	IE 4	Opera 12	Safari 3	WebView Android 1	Chrome Android 18	Firefox Android 4	Opera Android 12	Safari iOS 1	Samsung Internet Android 1.0
Custom text support 弃用非标	Chrome ? — 51	Edge No	Firefox ? — 44	IE Yes	Opera ? — 38	Safari ? — 9	WebView Android ? — 51	Chrome Android ? — 51	Firefox Android ? — 44	Opera Android ? — 41	Safari iOS No	Samsung Internet Android ? — 5.0

图例

完整支持
不支持
非标。预期跨浏览器支持较差。
弃用。不要用于新网站。

The HTML specification states that authors should use the Event.preventDefault() method instead of using Event.returnValue to prompt the user. However, this is not yet supported by all browsers.

When this event returns (or sets the returnValue property to) a value other than null or undefined , the user will be prompted to confirm the page unload. In older browsers, the return value of the event is displayed in this dialog. Starting with Firefox 44, Chrome 51, Opera 38, and Safari 9.1, a generic string not under the control of the webpage will be shown instead of the returned string. For example:

Firefox displays the string, "This page is asking you to confirm that you want to leave - data you have entered may not be saved." (see bug 588292 ).
Chrome displays the string, "Do you want to leave this site? Changes you made may not be saved." (see Chrome Platform Status ).

Internet Explorer does not respect the null return value and will display this to users as "null" text. You have to use undefined to skip the prompt.

In some browsers, calls to window.alert() , window.confirm() ，和 window.prompt() may be ignored during this event. See the HTML specification 了解更多细节。

Note also, that various browsers ignore the result of the event and do not ask the user for confirmation at all. In such cases, the document will always be unloaded automatically. Firefox has a switch named dom.disable_beforeunload in about:config to enable this behaviour. As of Chrome 60, the confirmation will be skipped if the user has not performed a gesture in the frame or page since it was loaded. Pressing F5 in the page seems to count as user interaction, whereas mouse-clicking the refresh arrow or pressing F5 with Chrome DevTools focused does not count as user interaction (as of Chrome 81).

元数据

最后修改： May 4, 2020

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句法

范例

注意事项

规范

浏览器兼容性

图例

相关话题